A thin film transistor (TFT) is a known device in which a thin film semiconductor is used. A TFT is composed by forming a thin film semiconductor on a substrate and using this thin film semiconductor for the semiconductor regions of a transistor. TFTs are used for various integrated circuits, and particularly for active matrix type liquid crystal display units and the like.
As the thin film semiconductor in a TFT, it is convenient to use an amorphous silicon film but there is the problem that they have poor electrical characteristics. Accordingly, a crystalline silicon film is utilized in order to improve the characteristics of a TFT. To obtain this crystalline silicon film, an amorphous silicon film is formed and then crystallized by a heat treatment.
However, the crystallization by heating has required a high temperature process at 600.degree. C. or higher, and in addition, it has been necessary to take 10 hours or longer. This has caused the problem that it is difficult to use an inexpensive glass substrate having a low distortion point.
Research carried out by the present inventors has shown that the addition of trace amounts of elements such as nickel, palladium and lead to an amorphous silicon film makes it possible to carry out crystallization by heat treatment of about 4 hours in a low temperature process of 550.degree. C. or lower. Further, it has become clear that also when the crystallization is carried out by laser, the same effect can be obtained.
However, the presence large quantities of impurities such as nickel in a semiconductor damage the device characteristics and reliability of a device using the semiconductor, and is undesirable. That is, elements such as nickel needed in crystallizing an amorphous silicon, are required not to be contained as far as possible in the crystalline silicon obtained.
Further, when the crystallization is carried out by laser, ridges of the crystal growth on a crystal surface in the form of projections are formed. Since the ridges have an influence on the flatness of a film surface, it is desirable that as far as possible they do not exist.